Several solutions have already been proposed for individualizing optical fibers in order to enable them to be identified in a ribbon or even in a bundle of fibers that are independent from one another.
In the summary of document JP-A-57040203 as published in the WPI database, an optical fiber is protected by an individualized colored primary covering and by a transparent secondary covering. In particular, the primary covering may be made of colored epoxy resin. The secondary covering includes an inside cushioning layer, in particular of transparent silicone resin, and an outside layer, in particular of transparent nylon.
The primary coverings of the various fibers are colored differently by using different dyes in the epoxy resin. Fibers differently colored in this way can be assembled together as a ribbon and they are then covered with a common cushioning layer and the above-mentioned outside layer of secondary covering. For splicing operations, the outside layer and the cushioning layer of the secondary covering are removed and the primary covering is itself eliminated by means of solvent-impregnated gauze.
That technique of individualizing fibers by coloring their primary coverings can pose problems when making the colored primary covering. An effect of adding the dye is to change the polymerization rate of dye-free or "natural" epoxy resin. The rate is slowed down, but above all it varies as a function of the color and of the uniformity of the various dyes. Consequently, the quality of polymerization becomes difficult to control. As a result it often happens that individualized fibers separate badly from the ribbon and/or that the fibers are badly stripped when it is necessary to perform a splicing operation, with this being due essentially to differences in the polymerization quality of the colored coverings which can cause traces or particles of colored matter be retained on some of the fibers.
In addition, the presence of dye in the primary covering, in particular when the dye is opaque and thus absorbent, can make it impossible during splicing operations to use local injection and detection systems with such fibers since such systems inject light into one of the fibers through its primary covering and transmitted light is similarly detected through the primary covering.
Document GB-A-2 109 581 teaches individualizing fibers that have been previously been protected by a primary covering and preferably also by a secondary covering by means of a colored ink coating the outside surfaces of the protected fibers. This is done when making a cable that is to include the fibers by causing each of the fibers to pass through an inking station prior to the fiber being installed in the cable. The ink used is chosen to have good covering properties and to be quick-drying.
Document FR-A-2 549 238 also relates to individualizing optical fibers. According to document, the outside surfaces of fibers that have already been protected are colored continuously by applying a colored outside layer on each of them, the dyes in said outside layer differing from one fiber another, and annular ink marks are also made one,he colored outside layer. The ink is in a color that contrasts with the possible colors for the colored outside layer, and may be black, for example.
The solutions known from the two last-mentioned documents enable fibers to be individualized when they are left independent of one another. Problems linked to a structure comprising a ribbon of fibers are not raised and are therefore not dealt with.